[hpsdr] To kit or not to kit...redux.

[Philip Covington]

Hi Ken,

On 8/15/06, Ken Klein <kenklein at austin.rr.com> wrote:
> ***** High Performance Software Defined Radio Discussion List *****
> Anyway, I got a 10X microscope; a very small tipped iron (the tiniest
> I could find) with heat control, some very tiny gauge solid core solder and
> some liquid solder flux.

The microscope is probably the best investment you can make in doing this stuff.

> I adjusted the iron to just melt the solder and a then some.  Sorry, but I
> don't have any way of knowing the tip temperature, but I know it's not very
> much above solder melting temp.  I don't think I'm in danger of damaging
> parts from excess heat.

For those looking for a soldering iron recommendation you can't go
wrong with the Weller WES51.  I think DigiKey sells it for about $120.
 Probably can get it cheaper elsewhere.  Another alternate might be
the Hakko 936 which sells for around $75.

> Anyway, the technique that worked the miracle is this:
>
> 1.      Wet the surface of the area with a very small amount of solder flux.
> I kept thinking I was using too much, but it didn't seem to matter much.

Really, there is no such thing as too much flux with hand soldering
SMT.  If you are worried that you are using too much flux it probably
means you are not using enough.  I hear about people having to remove
solder bridges on the fine pitch ICs with solder wick.  If the IC was
not drowning in flux before you soldered it you did not use enough
flux - this is the key to avoiding solder bridges on the fine pitch
stuff.  Using solder wick is just asking to pull up a pad or trace
unless you use a lot of flux with it - in which case you should have
used copious flux in the first place to avoid the solder bridges you
are trying to remove with the wick.

On the subject of flux:  The best advise I can give is to throw away
your rosin based flux and solder.  Get a gallon of  Kester 2331-ZX
Water Soluble Soldering Flux and some 0.015" Kester "331" Organic
Water Soluble Flux core solder (63/37).

Flux:
<http://www.philcovington.com/HPSDR/TOOLS/flux.JPG>

Solder:
<http://www.philcovington.com/HPSDR/TOOLS/solder.JPG>

Some people laugh when they see the gallon of flux (comments like: you
have 20 years supply there).  I laugh at those people when I hear
about them pulling up pads and traces with their solder wick.
Obviously they don't know that they are not using enough flux.

The water soluble stuff literally cleans up with running water.  No
scrubbing, no smelly chemicals...  Try it - you'll like it..

You can get some cheap syringes to dispense it:

<http://www.philcovington.com/HPSDR/TOOLS/dispense.JPG>

> 2.      Place the part EXACTLY where you want it, nudging it with tweezers
> until it is exactly in place.  The microscope works wonders here with my
> old, old eyes.
>
> 3.      Now, again under the scope of course, touch the tip of the iron to
> the PAD next to the leg of the device you are soldering being very careful
> NOT to touch the part itself.  If you do, it'll displace the part from where
> you want it to be.

I use these tweezers to hold the two lead devices while touching the
pad and one side of the chip:

<http://www.philcovington.com/HPSDR/TOOLS/picks.JPG>

The picks are nice for inspection on the fine pitch stuff.  You can
literally tell if a pin is soldered by the sound of the pick as you
drag across the pins.

I wish I had the proper video equipment to make a video of SMT
soldering techniques.  If a picture is worth a thousand words then a
video is worth 1 million.  I'd really like to demonstrate the
technique to hand solder a 208 pin 0.5mm pitch QFP in about 60
seconds.  I have had some people email me claiming it is not possible
without solder bridges...  I'll take bets on that any day!

73 de Phil N8VB

 1155822507.0